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<center><b>CHAPTER 3 - Labels & Addresses & Variables</b></center><br><br>


Okay, let's get to variables. In previous chapter i wrote that variable is
general term for space which stores some value. Registers are variables for
example. But there is limited number of registers (VERY limited, some 8 + few
special), and this is nearly always not enough. For this reason memory (RAM -
random access memory) is used.<br><br>

<b>NOTE:</b> when someone says "variable" he almost always means memory
variable.

<br><br><br>
<b>3.1. Labels</b>

<br>
Problem is that you have to know WHERE in memory is some value stored.
Position in memory (called "address") is given by number. But it is quite hard
to remember this number (address) for every variable.
<blockquote class="term">
term: <b>address</b><br>
number which gives position in memory
</blockquote>

Another problem with addresses is that when you change your program, address
can be changed too, and so you would have to correct this number everywhere
where it is used. For this reason addresses are represented by "labels".
Label is just some word (not string, it is not enclosed in apostrophes),
which, in your program, represents address in memory. When you compile your
program, compiler will replace label with proper address. Label consists of
alphabet characters ("a" to "z", "A" to "Z") numbers ("0" to "9"), underscores
("_") and dots ("."). But first character of label can't be number or dot.
Label also can't have same name as directive or instruction (instruction
mnemonics). Labels are case sensitive in FASM ('a' is NOT same as 'A').
Example of labels:
<br><br>

<table align=center width=70% border=0 bgcolor=#7F91EC cellspacing=1 cellpadding=0>
<tr align=center bgcolor=#CBCFD8><td><code>name       </code></td><td>is label</td></tr>
<tr align=center bgcolor=#CBCFD8><td><code>a	      </code></td><td>is label</td></tr>
<tr align=center bgcolor=#CBCFD8><td><code>A	      </code></td><td>is label, different from "a"</td></tr>
<tr align=center bgcolor=#CBCFD8><td><code>name2      </code></td><td>is label</td></tr>
<tr align=center bgcolor=#CBCFD8><td><code>name.NAME2 </code></td><td>is label</td></tr>
<tr align=center bgcolor=#CBCFD8><td><code>name._NAME2</code></td><td>is label</td></tr>
<tr align=center bgcolor=#CBCFD8><td><code>_name      </code></td><td>is label</td></tr>
<tr align=center bgcolor=#CBCFD8><td><code>_	      </code></td><td>is label</td></tr>
<tr align=center bgcolor=#CBCFD8><td><code>.name      </code></td><td>is not label, because is starts with dot (labels starting with
		     dot have special meaning in FASM, which you will learn later)</td></tr>
<tr align=center bgcolor=#CBCFD8><td><code>1	      </code></td><td>is not label because it starts with number</td></tr>
<tr align=center bgcolor=#CBCFD8><td><code>1st_name   </code></td><td>is not label for same reason</td></tr>
<tr align=center bgcolor=#CBCFD8><td><code>name1 name2</code></td><td>is not label, because it contains space</td></tr>
<tr align=center bgcolor=#CBCFD8><td><code>mov	      </code></td><td>is not label, because "mov" is instruction mnemonics</td></tr>
</td></tr></table>
<br>
<blockquote class="term">
term: <b>label</b><br>
Placeholder for some address, eg. placeholder for some number, because address is number. In FASM you can
use label same way as any other number (not really, but it doesn't matter for you too much now).
</blockquote>

You can define label using directive "label". This directive should be
followed with label itself (label name). For example:
<br><br>

<table align=center width=70% border=0 bgcolor=#7F91EC cellspacing=1 cellpadding=0>
<tr align=center bgcolor=#CBCFD8><td width=120><code>label name</code></td><td>is label definition, it defines label "name" </td></tr>
<tr align=center bgcolor=#CBCFD8><td width=120><code>label _name</code></td><td>is label definition, it defines label "_name"</td></tr>
<tr align=center bgcolor=#CBCFD8><td width=120><code>label label</code></td><td>is not label definition, because "label" can't be name of label
			     as decribed in previous paragraph
</td></tr></table>
<br>

this will define label that will represent address of data defined behind it<br><br>

<blockquote class="term">
directive: <b>label</b>
</blockquote>

<blockquote class="term">
<b>label definition</b><br>
directive <b>label</b> followed by label name
</blockquote>

Shorter way to define label is just writing label name followed by colon
(<code>:</code>)
<blockquote class="code"><pre>
name<font color=#333399>:</font>
_name<font color=#333399>:</font>
</pre></blockquote>
but we won't use this way for some time.
<br><br><br>

<b>3.2. Variable definition</b>

<br>
Now how we can return to problem with variables: how to define variable in
memory. Program you create (compiled program, in machine code) is loaded to
memory at execution time, where processor executes it instruction by
instruction. Look at this program:
<blockquote class="code"><pre>
org <font color=#339933>256</font>
mov al<font color=#333399>,</font><font color=#339933>10</font>
db <font color=#bb0000>'this is a string'</font>
int <font color=#339933>20h</font>
</pre></blockquote>

This program will probably crash, because after processor executes <code>mov
al,10</code> then it reaches string. But in program there is no difference between
string and instructions in machine code. Both are translated into array of numeric
values (bytes). There is no way processor can differ whether numeric value is
translation of string or translation of instruction. In this example,
processor will execute instructions whose numeric representation (in machine
code) is same as ASCII representation of string "this is a string".

Now look at this:
<blockquote class="code"><pre>
org <font color=#339933>256</font>
mov al<font color=#333399>,</font><font color=#339933>10</font>
int <font color=#339933>20h</font>
db <font color=#bb0000>'this is a string'</font>
</pre></blockquote>

This program will not crash, because before reaching bytes defined by string
processor reaches instruction <code>int 20h</code>, which ends execution of program. So
bytes defined with string will not be executed, it will just take some space.
This is way how you can define variable - define some data at place where
processor won't try to execute it (behind <code>int 20h</code> in this case).<br><br>

So code with byte-sized variable of value 105
<blockquote class="code"><pre>
org <font color=#339933>256</font>
mov al<font color=#333399>,</font><font color=#339933>10</font>
int <font color=#339933>20h</font>
db <font color=#339933>105</font>
</pre></blockquote>

Last line defines byte variable containing 105.<br><br>

Now how to access variable? First we must know address of variable. For this
we can use label (described above, reread it if you have forgotten)
<blockquote class="code"><pre>
org <font color=#339933>256</font>
mov al<font color=#333399>,</font><font color=#339933>10</font>
int <font color=#339933>20h</font>
label my_first_variable
db <font color=#339933>105</font>
</pre></blockquote>

So we already know address of variable. It is represented by label
<code>my_first_variable</code>. Now how to access it? You may think it is, for example
<blockquote class="code"><pre>
mov al<font color=#333399>,</font>my_first_variable
</pre></blockquote>

but no! Remember i told that label (<code>my_first_variable</code> in this
case) stands for address of variable. So this instruction will move address of
variable to <code>al</code> register, not variable's contents. To access
contents of variable (or contents of any memory location) you must enclose it's
address in brackets (<code>[</code> and <code>]</code>). So to access contents
of our variable, and copy it's value to <code>al</code> we use
<blockquote class="code"><pre>
mov al<font color=#333399>,[</font>my_first_variable<font color=#333399>]</font>
</pre></blockquote>

Now we will define two variables:
<blockquote class="code"><pre>
org <font color=#339933>256</font>
<font color=#333399>&lt;</font>some instructions<font color=#333399>&gt;</font>
int <font color=#339933>20h</font>
label variable1
db <font color=#339933>100</font>
label variable2
db <font color=#339933>200</font>
</pre></blockquote>

So to copy value of <code>variable1</code> to <code>al</code> we use
<blockquote class="code"><pre>
mov al<font color=#333399>,[</font>variable1<font color=#333399>]</font>
</pre></blockquote>

To copy <code>al</code> to <code>variable1</code> use
<blockquote class="code"><pre>
mov <font color=#333399>[</font>variable1<font color=#333399>],</font>al
</pre></blockquote>

To set value of <code>variable1</code> (exact: to set value of variable which is stored
at address represented by <code>variable1</code>) to 10 we could try
<blockquote class="code"><pre>
mov <font color=#333399>[</font>variable1<font color=#333399>],</font><font color=#339933>10</font>
</pre></blockquote>

but this will cause error (try it if you want). Problem is that you know that
you are changing variable at address <code>variable1</code> to <code>10</code>.
But what is size of variable? In previous two cases byte-size could be
determined because you used <code>al</code> register which is byte sized, so
compiler decided that variable at <code>variable1</code> is byte sized too,
because you can't move between operands with different sizes. But in this case,
value 10 can be of any size, so it can't decide size of memory variable. To
solve this we use "size operators". We	will talk about two size operators for
now: <code>byte</code> and <code>word</code>. You can put size operator before
instruction operand when accessing it to let compiler know what the variable
size is:
<blockquote class="code"><pre>
mov byte <font color=#333399>[</font>variable1<font color=#333399>],</font><font color=#339933>10</font>
</pre></blockquote>

Another way to make this is
<blockquote class="code"><pre>
mov <font color=#333399>[</font>variable1<font color=#333399>],</font> byte <font color=#339933>10</font>
</pre></blockquote>

in this case compiler knows that moved value 10 is byte sized so it decides
that variable is byte-sized too (because we can move byte sized value only to
byte sized variable).<br><br>

But it would be hard to always remember and always write size of variable when
you access it. For this reason you can assign size of variable to label when
you define it. Just write size operator behind label name in definition:
<blockquote class="code"><pre>
label variable1 byte
db <font color=#339933>100</font>
</pre></blockquote>

or
<blockquote class="code"><pre>
label variable1 word
dw <font color=#339933>1000</font>
</pre></blockquote>

now everytime you use <code>[variable1]</code> it will have same meaning as
<code>byte [variable1]</code> (or <code>word [variable1]</code> in second
example). So <code>mov [variable1],10</code> will work, in first case it will
store value 10 to byte at address <code>variable1</code>, in second case it
will store to word.

<blockquote class="term">
<b>size operator</b><br>
</blockquote>

<b>NOTE:</b> You can't move value between variables with different size:
<blockquote class="code"><pre>
mov byte <font color=#333399>[</font>variable1<font color=#333399>],</font> word <font color=#339933>10</font>
</pre></blockquote>
or
<blockquote class="code"><pre>
mov <font color=#333399>[</font>variable1<font color=#333399>],</font>al
...
label variable1 word
dw <font color=#339933>0</font>
</pre></blockquote>
<br>
<b>NOTE:</b> You can't access two memory locations in one instruction (except for
same special instructions). This is wrong, it won't be compiled:
<blockquote class="code"><pre>
mov <font color=#333399>[</font>variable1<font color=#333399>],[</font>variable2<font color=#333399>]</font>
</pre></blockquote>

use this:
<blockquote class="code"><pre>
mov al<font color=#333399>,[</font>variable2<font color=#333399>]</font>
mov <font color=#333399>[</font>variable1<font color=#333399>],</font>al
</pre></blockquote>

This will cause you some problems in the beginning but it will force you
to write faster code, and that is biggest reason to code assembly.<br><br>

<b>NOTE:</b> size operator assigned to label at definition has lower priority than
size operator before access to variable in instruction, so:
<blockquote class="code"><pre>
mov byte <font color=#333399>[</font>variable<font color=#333399>],</font><font color=#339933>10</font>
label variable word
dw <font color=#339933>0</font>
</pre></blockquote>

will access BYTE, while
<blockquote class="code"><pre>
mov <font color=#333399>[</font>variable<font color=#333399>],</font><font color=#339933>10</font>
</pre></blockquote>

will access WORD.<br><br>

I think you noticed that having two lines to define one variable is little too
much. There is a shorter way to define variable:
<blockquote class="code"><pre>
variable1 db <font color=#339933>100</font>
</pre></blockquote>

is same as
<blockquote class="code"><pre>
label variable1 byte
db <font color=#339933>100</font>
</pre></blockquote>

notice that size of variable is defined too. In general, if data definiton
(using <code>db</code> or <code>dw</code> directive) is preceded by label, then
it will define this label too, and assign size of defined data as size of
label.

It can be used with words too
<blockquote class="code"><pre>
variable2 dw <font color=#339933>100</font>
</pre></blockquote>

Some example of using variables:
<blockquote class="code"><pre>
mov ah<font color=#333399>,</font><font color=#339933>2</font>
mov dl<font color=#333399>,[</font>character_to_write<font color=#333399>]</font>
int <font color=#339933>21h</font>
int <font color=#339933>20h</font>
character_to_write db <font color=#bb0000>'a'</font>
</pre></blockquote>
<br><br>

<b>3.3. Addresses and basics of segmentation</b>

<br>
Now we will discuss addresses little more. I have told that address is number
(<b>!</b>) which gives some position in memory. You have learnt how to represent this
number with labels, so numeric addresses were maintained by compiler. But you
still don't know anything about format of this number. I will try to explain
it a little in this chapter.<br><br>

As you probably know, data in memory are stored in "bits" which can have value
0 or 1. You can consider memory as a (one dimensional) array of bits. 8
consecutive bits make one byte.  Address is number (index, position in array)
of byte. For example address "0" is address of first bit of memory (or address
of first byte), address "1" is address of eight bit (or address of second byte)
of memory etc. Easiest to comprehend is to take memory as (one dimensional)
array of bytes<br><br>

Address in .COM files is word-sized number, so
<blockquote class="code"><pre>
label var1
<font color=#333399>&lt;</font>some data<font color=#333399>&gt;</font>
mov al<font color=#333399>,</font>var1
</pre></blockquote>

is wrong. It may work if <code>var1</code> is lesser than 256 so it fits into
byte sized register, but in general, store addresses in word-sized variables,
we will talk about them little later.<br><br>

Now some examples on addresses. Check this file
<blockquote class="code"><pre>
label variable1
db <font color=#339933>10</font>
label variable2
db <font color=#339933>20</font>
label variable3
db <font color=#339933>30</font>
</pre></blockquote>

here address represented by <code>variable1</code> is 0, <code>variable2</code>
stands for 1, <code>variable3</code> is 2.<br><br>

OK, this looks nice but it is not true at all. Problem is that there are
usually more programs loaded in memory at same time (operating system, mouse
driver, you program etc.). When using this way, program would have to know
WHERE in memory will it be loaded so it can access it's variables. For this
reason addresses are "relative". It means that for every program that is
loaded is reserved some region in memory called "segment". All addresses in
memory accessed by this program is then relative to begginning of this area.
So <code>[0]</code> doesn't mean first byte of memory, but first byte of segment.

<blockquote class="term">
<b>segment</b><br>
consecutive region of memory reserved for one program
</blockquote>

How this works? Processor has few special registers (segment registers) which
holds address of segment (address of first byte of segment). Every time you
access memory in your program then contents of this segment register is added
to address given by you so <code>mov al,[0]</code> accesses first byte of your
segment.<br><br>

<b>NOTE:</b> I have told that memory addresses in .COM programs are words. That
means they can be in range 0 to 65535. So maximal size of one segment is
65536 bytes. This can be "tricked" by changing contents of segment registers,
but don't care about this now.<br><br>

<b>NOTE:</b> Segment is region in memory. But term "segment" is often used for
address of beginning of this region. Sad but true.<br><br>

So absolute address in memory has two parts: segment (exact: address of
beggining of segment) and second part, word sized value called "offset" which
is address relative to segment (address of beginning of segment).

<blockquote class="term">
<b>offset</b><br>
address relative to segment, or address "inside" segment. (first definition is more
exact, but second is easier to comprehend)
</blockquote>

<b>NOTE:</b> (important) I said labels represent address of variable. In fact,
labels in FASM represent offset of variable. That is why it called "flat"
(you will comprehend this later (much much later :))<br><br>

I won't get deeper into segment registers, how is address of begginning of
segment stored in them (there IS difference), take segment registers as some
kind of black box for now, it works and we can ignore it now.<br><br><br>

<b>3.4. 'org' directive explained</b>

<br>
As your program is loaded, it often needs some external info from program that
runned it. Best example is command line arguments, or it may need know WHO
runned him etc. This value must be, of course, stored in same segment in
program. In .COM files these data (passed to your program by program that
runned you) is stored in first 256 bytes of segment. So your program is loaded
from offset 256.<br><br>

<b>NOTE:</b> 256 byte structure in beginning of .COM file is called "PSP" which
stands for "program segment prefix"<br><br>

Now imagine this .COM program:
<blockquote class="code"><pre>
mov al<font color=#333399>,[</font>variable1<font color=#333399>]</font>
int <font color=#339933>20h</font>
variable1 db <font color=#339933>0</font>
</pre></blockquote>

(notice - no <code>org 256</code> directive). Instruction <code>mov al,
[variable1]</code> takes 3 bytes, <code>int 20h</code> takes 2 bytes, so
<code>variable1</code> will stand for offset 5. So instruction <code>mov
al,[variable1]</code> is <code>mov al,[5]</code>. So this instruction access
6th byte of segment (first byte is at offset 0). But I already told you that
in first 256 bytes of segment are stored some informations, and your program
is loaded behind them, from offset 256. So you don't want to <code>variable1</code>
to be 5, you want it to be 256+5. And this is what <code>org</code> directive
does. It sets "origin" of file addresses. <code>org 256</code> will tell FASM
to add 256 to offset held by every label defined behind this directive (before
next <code>org</code> directive). And this is exactly what we want in .COM
files.<br><br>

So code upwards won't access variable you want, it will access something in
PSP (first 256 bytes of segment). To make it work properly use:
<blockquote class="code"><pre>
org <font color=#339933>256</font>
mov al<font color=#333399>,[</font>variable1<font color=#333399>]</font>
int <font color=#339933>20h</font>
variable1 db <font color=#339933>0</font>
</pre></blockquote><br>

<b>NOTE:</b> <code>org</code> affects labels at time of defintion (for example
at <code>label variable byte</code> or <code>variable db 0</code>), not when
they are used (like at <code>mov ax,[variable]</code>). That means, that if you
change addresses "origin" with <code>org</code> directive after defining some
label, then label will still hold same value before and behind <code>org</code>
directive.  <br><br>

I won't tell you about data contained in PSP, you dont have to care about
them now.

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